Digital subscriber line (DSL) services, commonly referred to as “broadband” services, are deployed using metallic PSTN lines that run between a digital subscriber line access multiplexer (DSLAM) and modems in subscribers' properties. With asymmetric DSL (ADSL) the DSLAM is located in the exchange and the line can be typically up to 7 km in length. With very-high bit-rate DSL (VDSL), the DSLAM is located in a local cabinet with the line being much shorter, typically a maximum of 2 km. The line is normally made up of a twisted copper pair, but can include lengths of aluminium.
Faults on DSL lines are not uncommon, and currently most faults are found by customers reporting problems such as their line being noisy, having slower than expected broadband speed, or even interrupted broadband service. Troubleshooting a fault often includes performing line tests on the line. Line tests can also be performed proactively to identify faults before a customer reports them. These line tests are typically electrical line tests that measure the electrical characteristics of a line and check that the results meet a standard (for example, as set out in SIN349 by British Telecommunications plc). It is also possible to compare line tests over a period of time to see if the line's electrical characteristics are deteriorating. Once a fault has been detected, an engineer can use electrical line testing, typically pair quality tests, to try and determine where the fault is located and make the appropriate repairs.
However, there are a range of fault conditions which are not picked up by this process. This can be due to the faults being intermittent or not severe enough to be measureable using existing techniques. Intermittent faults are particularly problematic to find but can cause great disruption to broadband services where a line drop can result in a service outage whilst the line retrains.
Furthermore, the deployment of PSTN lines requires joints to be made to join together different sections of cable. Joints are weather sealed, typically using a combination of gel filled crimp connectors and/or physical enclosures such as joint boxes, manholes and cabinets, and insulated from each other. However, weather sealing can fail allowing moisture into joints causing corrosion or conductivity between pairs, commonly known as a wet joint. Insulation can also be damaged by being abraded against physical objects such as trees, resulting in moisture affecting the line.
International patent application WO2013/154579 describes various diagnostic methods for telephone lines. Patterns of line data are used, and observed data distributions are classified using modelled distributions previously determined to correspond to known line activity, fault type, or fault location.
“Management of a DSL copper network using built-up qualification tools” by Martin Nilsson, Master's Degree Project, Stockholm 2005, provides an overview of DSL systems, and introduces concepts for regular monitoring measurements on a line performed periodically. The data is collected in a database for further analysis. A toolbox of programs are described to put the concepts into action.